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\section{Introduction}
\label{sec:introduction}

One of the open questions of the standard model (SM) of particle physics is the strong CP problem, i.e. the question why quantum chromodynamics induces no experimentally observed CP-symmetry breaking. A possible solution to this problem was developed in 1977 by R. Peccei and H. Quinn~\cite{Peccei:1977hh} and involves the breaking of an additional ${U(1)}_A$ symmetry, leading to the appearance of a new pseudo-Nambu-Goldstone boson, known as axion \cite{Weinberg:1977ma}, \cite{Wilczek:1977pj}. It was hypothesized already in 1983 that the axion would be also a good candidate to explain the observed dark matter content of the universe \cite{Abbott:1982af}. 

There is a number of beyond SM (BSM) extensions, predicting very weakly interacting sub-eV particles (WISPs) at a low energy scale, having very similar properties to axions (in supersymmetric theories see \cite{Covi:1999ty}, in string theory see \cite{Svrcek:2006yi} and in the Conformal Standard Model see \cite{Meissner:2007xv}). All these BSM models predict a photon-photon-ALPs vertex ($\gamma\gamma a$) which enables experimental searches that are based on lasers, microwave cavities and strong electromagnetic fields. Several of these experiments have been proposed and implemented in the recent years \cite{Andriamonje:2007ew}, \cite{Ehret:2010mh}, \cite{Rybka:2010ah}. A detailed overview can be found in ~\cite{Jaeckel:2010ni}. 


The OSQAR (Optical Search for QED Vacuum Bifringence, Axions and Photon Regeneration) experiment, located at CERN, combines strong magnetic fields, that are provided by spare magnets of the large hadron collider (LHC), with a high intensity laser to search for WISPs at the low energy frontier. Its designed as a typical Light Shining Through Wall (LSW) experiment, first developed in 1993 ~\cite{Cameron:1993mr}, exploiting the di-photon coupling of a WISP field. 

In this paper, we report on the results obtained during the 2014 data-taking campaign of the OSQAR experiment. Compared to the previous measurement of OSQAR \cite{Pugnat:2013dha}, a new laser-system and a new CCD camera with an efficiency noise suppression was used. In addition, an improved data analyses strategy has been applied during the data analysis. These improvements result in an exclusion limit on the ALPs-photon-photon coupling of $\gagg < 3.2 \cdot 10^{-8} \GeV^{-1}$ in the massless limit, which is currently the most stringent constraint from LSW-type experiments.

The paper is structures as follows: In section \ref{sec:setup}, we describe the experimental setup and the data-taking procedure of the 2014 run. Special focus is drawn in section \ref{sec:analysis} on the analysis strategy of the resulting data. The corresponding limits are derived and presented in section \ref{sec:limits}. The paper concludes with section \ref{sec:conclusion}.
